Surface acoustic wave devices

ABSTRACT

The surface acoustic wave devices of the invention utilise the reflection of acoustic waves at gratings of irregularly spaced discontinuities, said devices comprising between an input (3) and an output (4) transducer, and gratings (2) of discontinuities, 3 dB directional couplers (5) separating the incident acoustic waves from the delayed reflected acoustic waves and making it possible to create dispersive delay lines and band-pass filters.

United States Patent [1 n1] 3,882,430 Maerfeld May 6, 1975 SURFACEACOUSTIC WAVE DEVICES [75] Inventor: Charles Maerfeld, Paris, FrancePrimary Exam" ler james,w' Lawrence Asszstant Exammer Marvm Nussbaum[73] ASSigneeI O SO -C a France Attorney, Agent, or FirmRoland Plottel,Esq. [22] Filed: June 25, 1974 [21] Appl. No.: 482,848

[57] ABSTRACT [30] Foreign Application Priority Data The surfaceacoustic wave devices of the invention June 29, 1973 France 73.23999utilise the reflection of acoustic waves at gratings of irregularlyspaced discontinuities, said devices com- [52] US. Cl 333/30 R; 310/8.1;310/98; prising between an input 3 and an Output 4 trans 333/72 ducer,and gratings (2) of discontinuities, 3 dB direc- [51] I Cl 9/26; H03h9/30; 7/00 tional couplers (5) separating the incident acoustic [58]Field of Search 333/30 R, 72; 310/81, waves from the delayed fl dacoustic waves and 310/827 making it possible to create dispersive delaylines and band-pass filters. [56] References Cited UNITED STATES PATENTS7 Claims, 3 Drawing Figures 3,836,876 9/1974 Marshall et al. 333/72 1SURFACE ACOUSTIC WAVE DEVICES The present invention relates to surfaceacoustic wave devices comprising a dispersive grating constituted byirregularly spaced surface discontinuities and imparting to the waveswhich it reflects, a delay which is a function of their frequencies. Theinvention relates more particularly to a novel method of injecting thewave into such devices, which makes it possible to simply andeffectively separate the incident wave from the delayed wave, whilstutilising a particularly simple structure in these devices.

The devices described in the present invention, comprise a block ofpiezo-electric material (or a block of some other material covered witha layer of piezoelectric material) capable of propagating surfaceacoustic waves generated in the piezo-electric material by an inputtransducer of comb-type for example. A grating of mechanical orelectrical discontinuities, mutually parallel and capable of reflectingacoustic waves selectively as a function of their frequencies, isarranged on the piezo-electric material. These discontinuities arearranged in such a fashion that the spacing between two neighbouringdiscontinuities increases or decreases as one moves away from thetransducer generating the acoustic wave in the device. The propagatedacoustic wave is reflected at one or the other of these discontinuities,as a function of its frequency.

Dispersive delay lines utilising reflection at such discontinuities, aredescribed for example in U.S. Pat. No. 3,400,341 filed in the UnitedStates on Mar. 8, 1966 and entitled Elastic dispersive delay line.

In the delay lines disclosed in this patent, the discontrnuities, whichare mechanical discontinuities constituted by marks engraved on theline, are disposed perpendicularly in the direction of wave propagation.In this case, the incident wave and the reflected wave P pagate alongthe same trajectory and the input transducer serves at the same time asan output transducer; in other words, this transducer receives thedeyed, reflected acoustic wave and converts it to an electrical wave.This arrangement requires the use of an associated circulator, outsidethe delay line, to separate the incident electrical wave being delayed,from the delayed reflected wave. It is clear that this kind of angementconsiderably increases the bulk and weight of the delay line as well asthe complexity of its Operation.

It rs well known to overcome this complication by p ating thetrajectories of the incident and reflected oustic waves, on the delayline, each of them then g espectively emitted and received by adifferent transduce A method which enables this kind of separation to beachieved, is disclosed in the aforementioned Ame patent. It consists inarranging, as before, discontinutres perpendicular to the length of thedelay line, but in this t me supplying to these discontinuities a wavehose direction of propagation is inclined in relation to saiddiscontinuities in order to produce an angle of incidence other than 90and to achieve reflection whic s symmetrical with the angle of incidenceand not coincrdental therewith,

{another method is described in an article by R- Wrllramson and enry L.Smith entitled Large-timebandWrth-product-surface-wave pulse compressorem- P t g reflective gratings, published in Electronic Letters ofAug.10, 1972, Volume 8, No. l6. This consists in using acoustic waves whichpropagate parallel to the length of the delay line but in having twoidentical discontinuity gratings arranged side by side on the line, thediscontinuities in each of the gratings, which are parallel to oneanother, being symmetrically inclined in relation to the direction ofpropagation of the waves. The waves emitted by the input transducertowards the first grating, are reflected at right angles towards thesecond grating which again reflects them, again at right angles, towardsan output transducer arranged on the line again in the neighbourhood ofthe input transducer.

The main drawback of these structures resides in the highly criticalchoice of the relative angles of inclination between the emission andreception channels and the discontinuities, this due to the anisotropicnature of the material used as the propagation medium.

According to the invention there is provided a surface acoustic wavedevice comprising a piezo-electric substrate for propagating acousticwaves, an input transducer for converting high frequency electricalwaves into acoustic waves for propagating at the surface of saidsubstrate, at least one dispersive grating of non-uniformly spaceddiscontinuities, and an output transducer for converting the acousticwaves when reflected by said grating, into electrical waves, saidgrating (2) discontinuities being disposed perpendicularly to thedirection of propagation of said acoustic waves, said device furthercomprising at least one 3 dB coupler (5) of the type having regularlyspaced parallel conductive strips arranged between said input (3) andoutput (4) transducers and said dispersive grating (2) said conductivestrips of said coupler (5) being parallel to said discontinuities ofsaid grating (2).

The function of separation between incident and reflected waves isachieved by the 3 dB coupler acting upon the acoustic waves themselves.The result is simplicity of structure whilst avoiding the need toutilise acoustic waves propagating in several directions along thegrating.

Moreover, the structure of the devices in accordance with the inventionmakes it possible to create dispersive lines of long delay by combining,on one of the same substrate, several dispersive gratings along whichthe acoustic waves successively propagate. It also makes it possible tocreate band-pass filters.

Other objects, features and results of the invention will becomeapparent from the ensuing description given by way of non-limitativeexample and illustrated by the attached figures in which:

FIG. 1 is a schematic plan view of a device constituting a dispersivedelay line;

FIG. 2 is a schematic plan view of a variant embodiment which makes itpossible to obtain a dispersive line of long delay;

FIG. 3 is a schematic plan view of a variant embodiment which makes itpossible to create a band-pass filter.

FIG. 1 schematically illustrates in plan, a device which performs thefunction of a dispersive delay line. This line is made up of a substrate1 of a piezo-electric material (or a substrate of a non-piezo-electricmaterial covered with a layer of piezo-electric material) upon thesurface of which there is arranged a grating 2 of discontinuities. Thesediscontinuities can either be mechanical and therefore consist of marksengraved in the matic in nature. The transducers 3 and 4, respectivelyemitter or input transducers and receiver or output transducers, arearranged at one end of the substrate on the piezo-electric material. Ina manner known per se they can either be soldered to one face of thesubstrate 1, parallel to the discontinuities of the grating 2 as shownin FIG. 1, or again, may, as FIG. 2 shows (transducers 3 and 4), consistof metal interleaved comb strips deposited upon the surface of thesubstrate 1,.the direction of the emitted acoustic wave, in both cases,being perpendicular to the discontinuities of the grating 2. In theexample shown here, the spacing between the discontinuities of thegrating 2 rises as a function of the distance of the emitter transducer3 so that the generated acoustic wave is reflected by the grating 2selectively as a function of its frequency, the trajectory which itfollows, before being reflected, being the longer the lower itsfrequency. The reflected wave follows the reverse trajectory of theincident wave.

It goes without saying that the invention applies equally to devices inwhich the dispersive grating 2 is .been quoted, these having beengivenfor example in the aforesaid American patent.

An absorbant load of known type and schematically marked by thereference 6, is arranged at that end of the substrate 1 opposite to theend carrying the transducers 3 and 4, in order to absorb any acousticwaves not reflected by the grating 2.

In accordance with the invention, the dispersive delay line thus createdcomprises a directional coupler 5 with parallel conductive strips,arranged on the surface of the piezo-electric material 1, between thetransducers 3 and 4, and the grating 2. A directional coupler I of thiskind is known per se; it is described in particular in French Pat. No.72.15968 filed on May 4, 1972 and published under N0. 2.135.303.

This patent describes various kinds of parallel directional couplers andin particular a 3 dB directional coupler. The delay line in accordancewith the present invention incorporates this kind of 3 dB coupler.

The coupler 5, as schematically shown in FIG. 1, comprises parallel,regularly spaced conductive strips or bands arranged on the substrate 1,parallel to the discontinuities of the grating 2. The number ofconductive strips making up the coupler as well as the spacing betweenthe strips, are chosen on the one hand in order to transmit the band ofacoustic frequencies being handled by the line, and on the other hand tocreate a 3 dB coupler. 7

Thus, the acoustic energy applied by the transducer 3,to the channel Aof the coupler 5, is equally split between the two output channels B andD of the coupler. The wave produced by the channel D is, as in aconventional 3 dB coupler, delayed by 17/2 in relation to the wavedelivered by the channel B. These two waves propagate along the grating2 where they are reflected and are recombined in the coupler 5, still inaccor-' dance with the law governing the operation of conventional 3 dBcouplers. In other words, the two waves reflected and transmitted by thecoupler 5 towards the channel A, are in antiphase whilst the two wavesreflected and transmitted towards the channel C are in phase. All thereflected energy is thus directed by the coupler towards the receivertransducer 4.

It should be noted that the two incident waves produced by the channelsB and D have a relative phase shift of 1r/2 over the whole length oftheir outward and return trajectory along the grating 2, so that if thediscontinuities in this grating are of electrical type it is necessaryto split the grating 2 into two separate parts corresponding in the onecase to the wave produced by the channel B of the coupler 5 and in theother case to the wave produced by its channel D. Y

By way of example, in a delay line in accordance with the inventionhaving a centre frequency of 200 MHz, made of lithium niobate andcomprising interdigital transducers with five electrodes of a totalwidth of pm, as well as a dispersive grating with 6,000 electrodes, the3 dB coupler utilised comprises 55 electrodes whose pitch is 7' urn.Aline of this kind makes it possible to achieve a delay of 30 us withinafrequency band of 50 MHz. i v i The structures in. accordance with theinvention are particularly advantageous in the production of dispel"sive lines having long delays and a small size. I

FIG. 2 illustrates schematically in plan, a delay line of this kind,producing a long delay, with which there are associated on one and thesame piezo-electric substrate 1, several elementary delay lines throughwhich 53. The acoustic wave furnished by the transducerv 3to' thechannel A of the coupler 51 is-applied, through the latters outputchannels B and D to the grating 21 which reflects it towards the coupler51, Thefchann1el. C of this coupler, which picks up al l the;energy:re-, v flected by the grating 21, in turn supplies the'cha'hn'e'lI A of the coupler 52. The coupler 52, through itsQot'ttput channels Band D supplies the discontinuity grata ing 22 the reflected energyfromwhich isdirected to wards channel C of the same coupler 52. Thischannel in turn supplies the input A, of the coupler 5 3which, throughits output channels B and D directs the acoustic wave towards the thirdgrating 23 of'discont-inuities. The energy reflected by this grating isfinally 1 directed towards the channel C of the coupler 53 and picked upby the output transducer 4.

Absorptive loads 61 and 62, identical to that 60f FIG. 1 are arranged atthe two ends of the's'ubstrate.

Thus, the delay introduced into the waves, propa-- gated by such a delayline, is equal to the sum of the delays produced by each of the threecombined elemen-.

tary lines.

It is not merely lengthened because, for awave of a given frequency, itis multiplied by three'in. relation to I the delay which would beobtained with a single dispersive grating, but in addition thedifferentiation of the delay as a function of the wave frequency isincreased. In other words, the gratings 21, 22 and 23 are all arrangedin the same fashion in relation to the incident waves which, in thethree gratings, first of all encounter the discontinuities which areclosest together. Thus, the three gratings act in the same sense todifferentiate the delays in the waves which they reflect.

Here, again, as in the case of FIG. 1, the three gratings 21, 22 and 23can be reversed, the closely spaced discontinuities then being locatednearer the three corresponding couplers 51, 52 and 53, than the widerspaced discontinuities.

The creation of this kind of long delay structure is the simpler if allthe discontinuities it incorporates are parallel with one another.

It will be clear that the delay introduced by the line can be stillfurther lengthened by combining with still more dispersive gratingscoupled to the preceeding ones by a corresponding number of 3 dBcouplers, the output transducer '4 being located after the last coupler.

It is also possible to modify the structure of FIG. 2 by replacing thedispersive grating 22 by a non-dispersive reflector device reflectingsurface waves, for example the reflectors represented in FIG. 18 ofFrench Pat. No. 72.15968 hereinbefore referred to. It is described inthis patent, in relation to FIG. 19 in particular, how the I assembly oftwo such U-shaped reflectors with a 3 dB coupler, constitutes achannel-changing reflector 'device; it is a device of this kind which isutilised in our invention in order, for example, to create the surfaceflected by said grating, into electrical waves, said grating (2)discontinuities being disposed perpendicularly to the direction ofpropagation of said acoustic waves, said device further comprising atleast one 3 dB coupler 5 of the type having regularly spaced parallelconductive strips arranged between said input (3) and output (4)transducers and said dispersive grating (2), said conductive strips ofsaid coupler (5) being parallel to said discontinuities of said grating2).

v2. A device according to claim 1, wherein said discontinuities of saiddispersive grating 2) are mechani- I (2) and one 3 dB coupler (5).

5. A device according to claim 1, comprising several dispersive gratingscoupled by 3 dB couplers, arranged between the input transducer (3) andthe output transducer (4), in the path of the acoustic waves whenpropagating, said acoustic waves, when delivered by said inputtransducer (3) successively passing through said dispersive gratingsbefore being picked up by said output transducer (4).

6. A device according to claim 5, for constituting a dispersive delayline, comprising the same number of dispersive gratings (21, 22, 23) andof 3 dB couplers (51, 52, 53), said dispersive gratings all beingarranged in the same fashion in relation to the 3 dB couplers forsupplying them with the acoustic wave, so that each of said dispersivegratings delays in the same way the waves of different frequency whichit reflects when 0pcrating.

31 and 32 are reversed in relation to one another so that thedifferences in delay introduced by the first, 31, I

into the waves of different frequencies, are compensated by the grating32, and so that the waves picked up by the output transducer 4 all havethe'same delay.

What is claimed is:

l. A surface acoustic wave device comprising a piezo-electric substratefor propagating acoustic waves, an input transducer for converting highfrequency electrical waves into acoustic waves for propagating at thesurface of said substrate, at least one dispersive grating ofnon-uniformly spaced discontinuities, and an output transducer forconverting the acoustic waves when re- '7. A device according to claim5, for constituting a 1 band-pass filter, comprising an even number ofsaid dispersive gratings (31, 32) each associated with a 3 dBcoupler(5l, 52), two gratings of a pair being reversed I in relation toone another so that the delay differences introduced by one of them intothe acoustic waves of fi plers combined with said dispersive gratings ofa pair,

themselves being coupled by a third 3 dB coupler (52) with which thereis associated a non-dispersive surface wave reflector 33, 34).

1. A surface acoustic wave device comprising a piezo-electric substratefor propagating acoustic waves, an input transducer for converting highfrequency electrical waves into acoustic waves for propagating at thesurface of said substrate, at least one dispersive grating ofnon-uniformly spaced discontinuities, and an output transducer forconverting the acoustic waves when reflected by said grating, intoelectrical waves, said grating (2) discontinuities being disposedperpendicularly to the direction of propagation of said acoustic waves,said device further comprising at least one 3 dB coupler (5) of the typehaving regularly spaced parallel conductive strips arranged between saidinput (3) and output (4) transducers and said dispersive grating (2),said conductive strips of said coupler (5) being parallel to saiddiscontinuities of said grating (2).
 2. A device according to claim 1,wherein said discontinuities of said dispersive grating (2) aremechanical discontinuities constituted by bands engraved at the surfaceof the piezo-electric material acting as the propagation medium for theacoustic waves.
 3. A device according to claim 1, wherein saiddiscontinuities of said dispersive grating (2) are electricaldiscontinuities constituted by conductive strips arranged on the surfaceof the piezo-electric material serving as the propagation medium for theacoustic waves and wherein said grating is in two parts, said conductivestrips being interrupted at their centres.
 4. A device according toclaim 1 for constituting a dispersive delay line, comprising onedispersive grating (2) and one 3 dB coupler (5).
 5. A device accordingto claim 1, comprising several dispersive gratings coupled by 3 dBcouplers, arranged between the input transducer (3) and the outputtransducer (4), in the path of the acoustic waves when propagating, saidacoustic waves, when delivered by said input transducer (3) successivelypassing through said dispersive gratings before being picked up by saidoutput transducer (4).
 6. A device according to claim 5, forconstituting a dispersive delay line, comprising the same number ofdispersive gratings (21, 22, 23) and of 3 dB couplers (51, 52, 53), saiddispersive gratings all being arranged in the same fashion in relationto the 3 dB couplers for supplying them with the acoustic wave, so thateach of said dispersive gratings delays in thE same way the waves ofdifferent frequency which it reflects when operating.
 7. A deviceaccording to claim 5, for constituting a band-pass filter, comprising aneven number of said dispersive gratings (31, 32) each associated with a3 dB coupler (51, 52), two gratings of a pair being reversed in relationto one another so that the delay differences introduced by one of theminto the acoustic waves of different frequencies which it reflects, whenoperating, are compensated by the other, the two said 3 dB couplerscombined with said dispersive gratings of a pair, themselves beingcoupled by a third 3 dB coupler (52) with which there is associated anon-dispersive surface wave reflector (33, 34).